(Suki koso mono no jouzu nare; “People are good at what they like.”)
The things that you enjoy, you’re going to more cheerfully spend more time and effort on, so your skill in those areas will improve more quickly than it would if you were less invested. It’s easiest to become skillful when you’re enthusiastic. As a result, there’s going to be a decent amount of overlap between the things you like and the things you’re good at.
More tricky old grammar at work here. We start with a noun phrase centered around 物 (mono), “thing.” Beginning students of Japanese are sometimes taught to use this term for “things” that exist in a concrete, physical way, and to use 事 (koto) for more abstract “things” like fact in “The fact that today is cold shouldn’t stop us from going outside.” But mono has historically been used very broadly to indicate concrete and conceptual things, people, places, even as a sort of euphemism for supernatural beings – the mono in mononoke, for example.
Anyway, that noun is modified by 好き (suki). In modern Japanese this is taught as a “na-adjective,” but language learners might be better served by thinking of it as a noun that can take on adjectival or adverbial functions with the right particles attached. Here they’re connected with こそ (koso), a linking particle (係助詞, kakarijoshi) with special properties and an emphatic function. If you see こそ, a decent English translation may use “especially” or the like to convey that emphasis.
The assembled noun phrase is topped off with の (no), which can be confusing until you realize that in old Japanese this particle could also function like the modern subject marker が (ga). The noun phrase is our subject, then, and we’re given a predicate as well: the old-fashioned copula なり (nari). But wait! こそ has special properties, and one of them is to throw connected words from sentence-final form into “realis” form (已然形, izenkei). And so nari becomes nare and the sentence ends, albeit in a nonstandard way.
A modern rendition might go 好きな事こそ上手である (suki na koto koso jouzu de aru).
Modern speakers may misinterpret the final なれ as an imperative form of the verb なる (naru, “to become”) and accordingly add the particle に (ni), making ～上手になれ; this is an error.
This saying may be contracted to 好きこそ物の上手 or even 好きこそ上手 without loss of meaning.
「ジョン君、料理が上手ね」 「アハハ、食べるのが好きだから」 「あぁ、好きこそ物の上手なれというもんね」
(“Jon-kun, ryouri ga jouzu ne.” “Ahaha, taberu no ga suki dakara.” “Aa, suki koso mono no jouzu nare to iu mon ne.”)
[“John, you’re a really good cook!” “It’s because I like to eat, ha ha.” “Yeah, they do say ‘You get good at what you like.’”]